Did vendors gave you information that total assembly Dm=0? They could do xj with Dm<>0.

PS:
I don't see any necessity of modeling such xj as complex model.
1) There's no data available to do it exactly.
2) What is the goal using complex model? To estimate tie strength? This does not make sense.
I think simple model is the best way (total stiffness and Dm). Tie strength is vendor's problem. They must provide allowable axial displacement to prevent any problems with xj. But it's just my opinion